Wind turbine rotor blades experience a significant level of dynamic loading during operation of the wind turbine installation to which the, or each, rotor blade is connected in use. Not only is the rotor blade rotated about an axis of a hub of the wind turbine installation such that the loading due to the weight of the rotor blade is constantly changing but the wind turbine installation is subjected to significantly varying forces due to variation in the wind loading exerted thereon.
The pitch of respective blades may be changed as necessary, for example to enhance the lift generated by the rotor blade in particular wind conditions. It is also desirable to control the pitch of the blades to minimise the impact of extreme loads that can be experienced by the blade, for example during gusting winds.
Rotor blades may be monitored during operation to track loading experienced by each respective blade and the hub, both in terms of aerodynamic forces and weight forces exerted thereon.
Structural health monitoring of the wind turbine installation can also be performed as loading on the rotor blades occurs. A history of loading experienced by the rotor blade can be ascertained and this history can be used to estimate the fatigue life and current structural status of the rotor blades so that failure of the blades can be avoided.
Conventionally, monitoring is undertaken by placing load sensors on a surface of a shell of the rotor blade. Sensors can be mounted on an external surface of the shell, in which case the sensors are exposed to the environment and may experience deterioration as a consequence. Alternatively, the sensors may be placed on an inner surface of the shell where they would be protected from the environment. However, sensors located in this way may still be damaged during maintenance of the wind turbine. Furthermore, accurate placement of the sensors is difficult to achieve and such inaccuracies may lead to erroneous data being collected.
It is desirable to provide a sensor which overcomes some of the aforementioned disadvantages thus improving the accuracy of monitoring of loads to which the rotor blade is exposed.